Electrophotographic developing systems are classified into a one-component developing system involving the use of toner alone and a two-component developing system involving the use of a mixture of toner and a magnetic carrier.
The two-component developing system provides a stable triboelectric charging characteristic and is advantageous for maintaining high image quality over a long time period as compared to the one-component developing system because of the following reason: the two-component developing system involves the use of the magnetic carrier, and hence the magnetic carrier often undergoes triboelectric charging with the toner. In addition, the two-component developing system is often used particularly in a high-speed machine because the magnetic carrier shows a high ability to feed the toner to a developing zone.
An iron powder or a heavy metal-based ferrite carrier has been conventionally used as a magnetic carrier for the two-component developing system. However, any such magnetic carrier is apt to cause the deterioration of a developer such as toner spent to the magnetic carrier or the deterioration of toner due to the embedment of an external additive because of the following reason: the magnetic carrier has a large specific gravity and a high magnetization, and hence a magnetic brush becomes rigid.
In view of the foregoing, a magnetic carrier with its volume magnetization specified to 20 to 60 emu/cm3 (see JP 09-281805 A) and a resin-coated magnetic carrier with its average particle diameter and specified intensity of magnetization in 79.6 kA/m (1 kOe) (see JP 2002-91090 A) have been proposed for the purpose of controlling the magnetic force of a magnetic brush. Each of those proposals describes that an improvement in image quality by virtue of an improvement in dot reproducibility, and image stability over a long time period can be achieved by the following procedure: magnetic brushes on a developer carrying member are controlled so as to be soft and dense to such an extent that the adhesion of the carrier to an electrostatic latent image bearing member can be suppressed. However, when the triboelectric charge quantity of toner increases under a low-humidity environment, counter charge remaining on the magnetic carrier also increases, and hence it may become impossible to suppress the adhesion of the carrier sufficiently at the time of the application of an alternating electric field.
Meanwhile, investigations have been conducted on a resin-loaded magnetic carrier obtained by loading a resin into voids in the core of a magnetic carrier for the purpose of reducing the specific gravity of the magnetic carrier. For example, Japanese Patent Application Laid-Open No. 2007-57943 achieves the reduction in specific gravity with a magnetic carrier having the following structure: a resin is loaded into voids in a porous ferrite core characterized in that each void continuously reaches the inside of the core from the surface of the core so that a resin layer and a ferrite layer may be alternately present. However, when a starting material is nonuniform, even a magnetic carrier obtained by sufficiently loading a resin into a ferrite having voids formed of the starting material may be unable to obtain a strength enough for the magnetic carrier to withstand a stress which the magnetic carrier receives at the time of the mixing and stirring of a developer.
In view of the foregoing, Japanese Patent Application Laid-Open No. 2007-34249 proposes a magnetic carrier which has improved abrasion resistance while achieving a reduction in its specific gravity on the basis of the following procedure: a ferrite core having a fine porous structure is formed, and, furthermore, a non-magnetic oxide is incorporated into the core to impart a strength to the core.
A stress to be applied to the magnetic carrier at the time of the mixing and stirring of a developer is alleviated. In addition, the abrasion resistance of the magnetic carrier itself is improved. As a result, the magnetic carrier allows high-speed development and maintains image stability over a long time period, and the extension of the replacement life of the magnetic carrier is achieved. However, the non-magnetic oxide has a high resistance, and the resistance of the magnetic carrier eventually increases. As a result, the developer containing the magnetic carrier may be poor in developing performance particularly under a low-humidity environment, and the insufficient developing performance may lead to image defects such as blank dots.
In addition, Japanese Patent Application Laid-Open No. 2007-218955 proposes a carrier using a porous core having the following characteristics: the pore volume of the porous core is specified, the carrier core, that is, the porous core, is subjected to a resistance-increasing treatment to have a resistance high enough to suppress breakdown at the time of the application of a high voltage, and the porous core is excellent in spent resistance. The use of the carrier suppresses spent and provides a high-quality image. However, a developer containing such carrier core of which has an increased resistance may be poor in developing performance. As a result, even when an image density is sufficient, the following image defects (hereinafter referred to as “blank dots”) may occur: toner at the rear end of a halftone image portion is scraped at a boundary between the halftone image portion and a solid image portion, with the result that a white stripe is formed to highlight the edge of the solid image portion. Further, when the triboelectric charge quantity of the toner is increased so that fogging may be additionally alleviated and dot reproducibility may be additionally improved, an additional improvement in developing performance is requested, and hence an improvement in developing performance based on modifications to the carrier is requested.
As described above, investigations have been conducted on a method of alleviating a stress to be applied to a magnetic carrier at the time of the mixing and stirring of a developer in order that the deterioration of the developer may be prevented, and an improvement in image quality and an increase in speed at which development is performed may be achieved. Accordingly, a two-component developer capable of satisfying developing performance and durability under various environments has been demanded.